This patch also removes legacy support for @USER@. Only %USER% is valid
now.

There are two new parameters submitted that contain the currently
selected namespace and page id so it can be selected again
indenpendently from the opened namespace.

Previously the content of expanded namespaces was displayed below
all other pages, now it is placed below it's parent namespace.
The new comparison function is quite complex. Please have a look at it
if it can done easier or if there are cases I haven't considered where
the function produces incorrect results.

This patch moved the place where DOKU_PLUGIN is defined. It no longer
can be set from a normal config (only via preload)